This invention relates generally to filter media and more particularly concerns melt-blown filter media for use in HEPA and pre-HEPA filters.
A HEPA filter has a filtration efficiency of at least 99.97% to 0.3 micron particles. The efficiency of HEPA filters is measured in accordance with test procedures described in Military Standard 282, Test Method 102.1, using dioctylphthalate particles that average 0.3 micron at a face velocity of 10.4 to 10.5 feet per minute. The efficiency of the filter refers to the percentage of particles that are filtered out of the air stream by the HEPA filter. Filters are classified as HEPA only if they achieve the requisite 99.97% filtration efficiency. Filters having efficiencies from about 90 to 99.97% are referred to as pre-HEPA filters.
HEPA and pre-HEPA filters are used to filter air in clean rooms where integrated circuits and precision equipment are manufactured. HEPA and pre-HEPA filters are also used in filtering air for operating rooms to filter out bacteria and other contaminants which may be present in the air and harmful to patients.
Not only must HEPA and pre-HEPA filters provide the requisite filter efficiency, it is likewise important that the pressure drop across the filter be maintained as low as possible for a given filter efficiency. If the pressure drop becomes excessive across the HEPA or pre-HEPA filters, larger more powerful fans will be required to compensate for the excess pressure drop with the resulting increase in power and noise. Therefore, it is important that HEPA and pre-HEPA filters maintain the lowest possible pressure drop at a given efficiency rating.
Typically HEPA and pre-HEPA filter media are produced from glass filaments which filaments range in size from 0.3 to 2.0 microns. Glass filter media are formed in sheets by a wet (papermaking) process. In order for glass fiber HEPA and pre-HEPA filters to perform at the requisite efficiencies, the filament sizes must be small to yield pore sizes within the filter that are sufficiently small to assure that the 0.3 micron particles do not pass through the filter media. Because the wet process for making glass filter media is both time consuming and expensive, melt-blown materials appear to offer attractive possibilities for making filter media and perhaps HEPA and pre-HEPA filter media.
The melt-blown process is well known and is described in various patents and publications, including NRL Report 4364, "Manufacture of Super-Fine Organic Fibers" by V. A. Wendt, E. L. Boon, and C. D. Fluharty; NRL Report 5265, "An Improved Device for the Formation of Super-Fine Thermoplastic Fibers" by K. D. Lawrence, R. T. Lukas, and J. A. Young, and U.S. Pat. No. 3,849,241 issued Nov. 19, 1974, Buntin, et al.
Melt-blown materials made in accordance with conventional practice for use as wiping cloths and the like have been found to be useful as filter media, but such filter media generally have efficiencies well below 70%.
Buntin et al. U.S. Pat. No. 3,849,241 discloses a process for making melt-blown polypropylene mats which are said to be useful as filter media. The melt-blown process disclosed illustrates the inter-relationships between the apparent viscosity of the resin, the resin flow rate, and the gas flow, with particular emphasis on the elimination of shot by lowering the apparent viscosity of the resin. The process is said to produce mats having fibers with diameters from 0.5 micron to 5 microns and usually from 1.5 microns to 4 microns. There is no disclosure, however, that the resulting mats will perform as HEPA or pre-HEPA filter media.
Pall U.S. Pat. No. 4,032,688 discloses a continuous production process for making non-woven thermoplastic webs in cylindrical or sheet form, which webs are useful as filter elements. The webs disclosed are of melt-blown polypropylene and are deposited on a mandrel to form a cylindrical filter element. A sheet is made by drawing the cylindrical web off of the end of the mandrel and flattening it between rollers. The fiber diameters are disclosed to be less than 10 microns and preferably less than 5 microns in diameter. The melt-blown process of Example 1 is carried out by extruding polypropylene at a rate of 1.1 lbs. per inch of die width per hour with air flow (11 to 16 psi) adjusted to produce a fiber of 4 microns in diameter. The resulting filter medium is said to have a uniform pore size of 15 microns, but the resulting filter media do not appear to be HEPA or pre-HEPA filter media.
Perry U.S. Pat. No. 4,211,661 discloses a filter medium of spun-bonded polypropylene laminated to a melt-blown polypropylene sheet which filter element is used as a particle filter for filtering liquid coolant. The melt-blown polypropylene forms the downstream side of the substrate and provides a thin fibrous downstream layer capable of retaining and filtering fine particles, particularly fine metal particles.
Fukuda et al. U.S. Pat. No. 4,181,513 discloses an absorptive filter material having deodorizing, degassing, and decolorizing properties. The reinforcing sheet used in connection with the filter may be comprised of polypropylene fibers and is said to have dust-collection efficiencies of not less than 60%. There is no specific disclosure, however, of the polypropylene melt-blown material or its efficiencies as a filter element in the Fukuda et al. patent.
Wadsworth et al. U.S. Pat. No. 4,375,718 discloses a process for manufacturing an electrostatically-charged filter medium. The fibers in the filter medium may be polypropylene and may be formed by melt-blowing techniques. The melt-blown fiber sizes are disclosed to be from 0.3 to 5 microns in diameter. The melt-blown filter medium is then plied on either side with a contact web having specific electrical properties which will accept an electrical charge. The contact webs are made from cotton, rayon, or mixtures of cotton and rayon with wood pulp or other fibers such as hemp. The filter medium, however, has a low efficiency (58%) when used without an electrical charge.
Kubik et al. U.S. Pat. No. 4,215,682 discloses a melt-blown fibrous electret which is continuously formed by passing melt-blown fibers through an electrical field upon exit from a die tip. The resulting web is said to then possess a permanent charge. Examples 1-8 disclose melt-blown micro-fibers prepared from polypropylene which are 25 microns or less in average diameter. It appears that the resulting electrostatically-charged filter media have efficiencies of up to 68%, but the same material with no charge only is 45% efficient.
Carey U.S. Pat. No. 4,011,067 discloses a filter medium produced from either melt-blown or solution-blown micro-fibers. The filter medium is a plied medium having a base porous web, an intermediate layer of micro-fibers, and a top porous web. The outside layers contribute only a minor portion, normally less than 20% of the pressure drop, and are typically non-woven fibrous webs such as polyethelene terephthalate. The intermediate layer of micro-fibers is of sufficient thickness to produce a HEPA filter which will typically exhibit a pressure drop on the order of 3 to 4 inches of water at a face velocity of 50 feet per minute.
The prior art thus fails to disclose an uncharged, melt-blown polypropylene material which is useful as a HEPA or pre-HEPA filter medium.